In automobiles and home electric appliances, power ICs (Integrated Circuits) have been widely used so as to control voltage and current. Some power ICs use an MOSFET for power control (hereinafter referred to as a power MOSFET). On such a power IC, there are cases where a similar MOSFET (hereinafter referred to as a sense MOSFET) with a smaller area than the power MOSFET is mounted in order to detect (observe) current of the power MOSFET.
Generally, the power MOSFET has a configuration in which a plurality of small-sized MOSFETs (cells) are connected in parallel. The sense MOSFET also has a configuration in which a plurality of such cells are connected in parallel (refer to Patent Documents 1 to 3, for example). When drain-source voltages are set to be equal between the power MOSFET and the sense MOSFET, drain-source voltages between each cell that forms the power MOSFET and each cell that forms the sense MOSFET become equal. The same holds true for gate-source voltages. Accordingly, a mutually equal amount of current flows through each cell of the power MOSFET and each cell of the sense MOSFET. Thus, when a ratio of the number of the cells of the power MOSFET to the number of the cells of the sense MOSFET is n to one, a ratio of drain current of the power MOSFET to drain current of the sense MOSFET also becomes n to one.
As shown in FIG. 4, Patent Document 1 discloses a current detection circuit with a load 104 connected in series with a source terminal of a high side power MOSFET 101. Drain terminals of the power MOSFET 101 and a sense MOSFET 102 are both connected to a terminal 111. Gate terminals of the power MOSFET 101 and the sense MOSFET 102 are both connected to an input terminal 109 through a resistor 108. A control voltage is supplied to this input terminal 109. A source terminal of an MOSFET 106 is connected to a source terminal of the sense MOSFET 102.
A drain terminal of the MOSFET 106 is connected to a measuring resistor 105 through a terminal 110. A fixed potential is supplied to an other terminal of the measuring resistor 105. A gate terminal of the MOSFET 106 is connected to an output terminal of a differential amplifier 103. This differential amplifier 103 has two input ends. An inverted input end (−) of the differential amplifier 103 is connected to the source terminal of the sense MOSFET 102, while a non-inverted input end (+) of the differential amplifier 103 is connected to the source terminal of the power MOSFET 101. A voltage Vbb is applied to the terminal 111.
When a control voltage is supplied to the input terminal 109, the power MOSFET 101 and the sense MOSFET 102 are both brought into a conducting state. Then, load current flows through the load 104. Current flows through the sense MOSFET 102, MOSFET 106, and measuring resistor 105 as well. Now, assume that a source-drain voltage of the sense MOSFET 102 is larger than a source-to-drain voltage of the power MOSFET 101. In that case, a resistance of the MOSFET 106 increases due to an output voltage of the differential amplifier. Then, the current that passes through the sense MOSFET 102 is reduced, and a drain-source voltage of the sense MOSFET 102 is thereby reduced.
The current that passes through the sense MOSFET 102 is reduced until a difference between input voltages to the differential amplifier 103 becomes zero, or until a potential at the source terminal of the power MOSFET 101 equals a potential at the source terminal of the sense MOSFET 102. This means that in a stationary state, a current proportional to a current of the load 104 flows through the measuring resistor 105. That is, when an impedance of the load 104 is changed, control is performed so that a drain-source voltage of the power MOSFET 101 increases (or is reduced), the resistance of the MOSFET 106 is thereby reduced (or increases), and then the difference between the input voltages to the differential amplifier 103 always becomes zero.
[Patent Document 1] JP Patent Kokai Publication No. JP-A-8-334534
[Patent Document 2] JP Patent No. 2628694
[Patent Document 3] JP Patent Kokai Publication No. JP-A-11-68533